Wired telecommunications of today relies more and more on telephone lines to facilitate data and voice transmissions. From computer modems and fax machines to video players, the telephone line has played an important role in making the world smaller via telecommunications. Because of the proliferation of uses for telephone lines, there has been a growing concern to protect operating personnel and connected communications equipment from excessive voltages and currents that may occur on the lines, as typically caused by lightning strikes, power line crosses, and currents induced from adjacent power lines.
Primary telecommunications protectors, at a minimum, provide overvoltage protection. This is typically done with at least one protection element that is inserted between the tip and ring conductors of the telecommunications line and ground. When hazardous overvoltages are present on the line, these overvoltage protection elements change from a high impedance to a low impedance state, effectively shorting the hazardous overvoltages, and their associated overcurrents to ground and away from equipment and personnel. Once the hazardous overvoltage condition has subsided, these protection elements typically return to their original high impedance state. Some conventional overvoltage protection elements include thyristors, gas tubes, air-gaps, diodes, metal oxide varistors, and combinations of the above in various configurations to better provide overvoltage protection.
There are occasions when an excessive overcurrent may be present with no overvoltage. This is typically called a "sneak current" and may occur when there is AC induction on the line or somehow the tip and ring conductors are shorted or nearly shorted to ground. During such a condition the overvoltage protection element may not short to ground, thereby allowing hazardous overcurrents to pass by the protector to the equipment and personnel inside the house, building, or remote cabinet. This condition may damage telecommunications equipment over time or may cause a fire hazard. Current methods of stopping or diverting sneak currents and overcurrents in telecommunications applications typically employ heat coils or Positive Thermal Coefficient (PTC) elements. A heat coil typically comprises resistive elements that are connected in series with the wires of the line. When a sustained overcurrent exists, the heat caused by I.sup.2 R activates a thermally sensitive shorting mechanism, thereby permanently shunting current to ground and diverting such current away from the connected equipment and personnel. A PTC element also performs the function of sensing current on the line, however, it often prevents an overcurrent by switching into a high-resistance mode to stop the current from flowing as opposed to grounding the line, and thereby creating an open circuit upon reaching a predetermined overcurrent-induced temperature threshold.
While heat coils and PTC devices have proven to protect telecommunications lines from extended overcurrents of a certain magnitude, they are not true "current limiters," instead they limit the duration an overcurrent is allowed to exist on a line. In the case of heat coils, the existence of an overcurrent on the line may result in excessive heat buildup on the line and can actually cause a fire. Furthermore, because heat coils and PTC elements are thermal devices, they are sensitive to ambient temperatures and can be slow to operate. Of the two types, only PTC elements are self-resetting. They often require many seconds to operate, and depending on the magnitude and duration of the current surge, they normally require just as long to return to a low resistance state, thereby disturbing customer services for much longer than the actual overcurrent duration. Heat coils by nature of design also take several seconds to minutes to operate and are not resettable; therefore, they are ineffective against current surges, and they further disturb customer services after operation until they are replaced. Accordingly, a need exists for a reliable and fast automatically-resetting protection arrangement in telecommunications applications.